#include <stdio.h>
void result(char s[]) {
int i = 0;
int counter[26] = {0};
int number;
while (s[i] != '\0') {
if (s[i] >= 'A' && s[i] <= 'Z') {
number = s[i] - 'A';
counter[number]++;
} else if (s[i] >= 'a' && s[i] <= 'z') {
number = s[i] - 'a';
counter[number]++;
}
i++;
}
int max = -1;
for (int j = 0; j < 26; j++) {
if (counter[max] < counter[j]) {
max = j;
}
}
if (max > -1) {
printf("%c", max + 'a');
} else {
printf("-");
}
}
int main() {
char a[1001];
scanf("%[^\n]", a);
result(a);
return 0;
}